This invention relates to the preparation of coated aluminum-alloy articles, and, more particularly, to the preparation of coated and heat-treated aluminum rivets.
Fasteners are used to mechanically join the various structural elements and subassemblies of aircraft. For example, a large transport aircraft typically includes over one million fasteners such as bolts, screws, and rivets. The fasteners are formed of strong alloys of metals such as titanium, steel, and aluminum alloys. In most cases, the fasteners are heat-treated, as by an aging treatment, to achieve as high a strength, in combination with other desirable properties, as is reasonably possible for that particular alloy. Heat-treating usually involves a sequence of one or more steps of controlled heating in a controlled atmosphere, maintenance at temperature for a period of time, and controlled cooling. These steps are selected for each particular material in order to achieve its desired physical and mechanical properties.
It has been the practice to coat some types of fasteners with organic coatings to protect the base metal of the fasteners against corrosion damage. In the usual approach, the fastener is first fabricated and then heat-treated to its required strength. After heat-treatment, the fastener is etched with a caustic soda bath to remove the scale produced in the heat-treatment. Optionally, the fastener is alodined or anodized. The coating material, dissolved in a volatile carrier liquid, is applied to the fastener by spraying, dipping, or the like. The carrier liquid is evaporated. The coated fastener is heated to elevated temperature for a period of time to cure the coating. The finished fastener is used in the fabrication of the structure.
This coating approach works well with fasteners made of a base metal having a high melting point, such as fasteners made of steel or titanium alloys. Such fasteners are heat-treated at temperatures well above the curing temperature of the coating. Consequently, the curing of the coating, conducted after heat-treating of the fastener is complete, does not adversely affect the properties of the already treated base metal.
On the other hand, aluminum alloys have a much lower melting point, and thence a generally much lower heat-treatment temperature, than steel and titanium alloys. It has not been the practice to coat high-strength aluminum-alloy fasteners with curable coatings, because it is observed that the curing treatment for the coating can adversely affect the strength of the fastener. The aluminum-alloy fasteners are therefore more susceptible to corrosion than would otherwise be the case. Additionally, the presence of the organic coating aids in the installation of the fastener for titanium alloys and steel. The absence of the coating means that aluminum fasteners such as rivets must be installed using a wet sealant compound for purposes of corrosion protection. The wet sealant compound is messy and difficult to work with, and may require extensive cleanup of the area around the fastener using caustic chemical solutions.
There exists a need for an improved approach to the protection of aluminum-based fasteners such as rivets. The present invention fulfills this need, and further provides related advantages.